A data transfer protocol may be desired to provide an internal data interface between modules within a SONET element. Further, for a network element having east and west high-speed interfaces, a cross-connect and drop modules, it may be desired that the data entering the cross-connect from the high-speed interfaces and the drop modules be frame aligned to simplify the cross-connect implementation. It may also be desirable that an error checking mechanism exist in the frame structure.
For SONET signals incoming to and outgoing from the network element, the American National Standard T1.105-Draft-September 1990, incorporated herein by reference, defines a standard format including section, line and path overhead bytes B1, B2, B3, respectively, in Secs. 9.2.1, 9.2.2 and 9.2.3 thereof.
One byte (B1) is allocated in each STS-1 for a section error monitoring function. This function is a bit interleaved parity 8 (BIP-8) code using even parity. The section BIP-8 is calculated over all bits of the previous STS-N frame after scrambling. The computed BIP-8 is placed in the B1 byte of STS-1 No. 1 before scrambling. This byte is defined only for STS-1 No. 1 of an STS-N signal.
One byte (B2) is allocated in each STS-1 for a line error monitoring function. This function is also BIP-8 code using even parity. The line BIP-8 is calculated over all bits of the line overhead and STS-1 envelope capacity of the previous STS-1 frame before scrambling (total 801 bytes). The computed BIP-8 is placed in the B2 byte of the STS-1 before scrambling. This byte is provided in all STS-1 signals within an STS-N signal. The N-line BIP-8 bytes in an OC-N signal are intended to form a single error monitoring facility capable of measuring error rates up to 10.sup.-3 independent of the value of N. The parity errors detected by the N BIP-8 detectors, or equivalently, the single BIP-8 N detector, should be accumulated into a single error count for the OC-N line.
One byte (B3) is allocated for a path error monitoring function. This function is also a BIP-8 code using even parity. However, the path BIP-8 is calculated only over all bits of the previous STS SPE before scrambling (total 783 bytes).
Although these line error monitoring functions may be appropriate for line monitoring, they are not necessarily effective for satisfying a need which exists for efficiently monitoring transport of data and overhead between modules within network elements.